Protection of alternating current electric power systems



P. H. LIGHT Aug. 10, 1943.

PROTECTION OF ALTERNATING CURRENT ELECTRIC POWER SYSTEMS Filed April 24, 1942 rm n oEwr tL o nuam a A mW s n i 1P Patented Aug. 10, 1943 PROTECTION OF ALTERNATING CURRENT ELECTRIC POWER SYSTEMS Philip H. Light, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 24, 1942, Serial No. 440,277

12 Claims.

My invention relates to improvements in the protectionof alternating current electric power systems, and especially systems of the type wherein a fault to ground on a phase conductor of the system substantially increases the capacitance current to ground of the ungrounded phase conductors and wherein ground faults of a transient character are to be cleared by a zero sequence inductive connection to ground proportioned to provide on the occurrence of a ground fault on a phase conductor of the system a lagging current for effectively suppressing the capacitance current at the grounded point.

In alternating current power systems which operate without a neutral grounded more or less directly, single conductor-to-ground faults of a transitory or arcing character may be suppressed by a ground fault neutralizing device, as disclosed in United States Letters Patent 1,537,371, issued May 12, 1925, without interruption of service. Since a large percentage of the faults on alternating current electric systems involve initially, at least, only a single conductor-to-ground, such neutralizing device materiall improves the service continuity factor. The neutralizing device is usually connected between a neutral of the system and ground and proportioned to have, together with the means providing the neutral, a zero sequence inductance such as to provideon the occurrence of a ground on a phase conductor of the system a lagging current for effectively suppressing the capacitance current to ground of the ungrounded phase conductors. Usually the neutral is provided by inductive apparatus such as'a zig-zag or Y-delta grounding transformer or a Y-delta power transformer. Ordinarily the amount of the inductance provided by such transforming means in the neutralizer connection is a relatively small proportion of the total inductance of this connection.

In order to take care of faults which the ground fault neutralizer cannot suppress, a low impedance connection to ground may be established after the flow of lagging current in the neutralizer connection for a predetermined time without suppressing the ground fault, as disclosed in United States Letters Patent 1,378,577,, issued May 17, 1921. This low impedance connection, which is usuall effected by short-circuiting all or part of the ground fault neutralizer, allows suflicient flow of fault current for the intended operation of the ground fault responsive relays with which the system is usually provided to clear ground faults of a permanent character, because of the relatively low impedance of the transformer used to provide the neutral for grounding. In other words, when this low impedance connection is established, there is a relatively heavy current flow in comparison with the arc-suppressing lagging current flow.

Although it is economically feasible to provide a transformer with a relatively long time rating on the basis of the arc-suppressing lagging current flow, the cost of a transformer which would safely pass the relatively high current flow required for satisfactory ground fault relaying for anything but a short time rating is ordinarily prohibitive. Obviously then, if the ground fault relays should fail to function within the safe current-time rating of the transformer for this higher current flow, destruction of the transformer would almost certainly follow. If the ground connection of the transformer neutral is interrupted to prevent overheating of the transformer, then the system becomes an isolated neutral system with a ground fault on one phase. This is likely to result in dangerous overvoltages which would damage or destroy insulation and lightning arresters on the system. Also, to open the neutral this way requires additional costly equipment. Furthermore, the operating personnel is handicapped by the inability of determining where the fault is located.

One object of my invention is to provide an improved arrangement for protecting an alternating current electric system so that ground fault neutralizers may safely be used in connection with transformers which have a relatively short time rating for currents of the magnitude necessary for the groundfault relaying of the system without endangering the insulation of the system or apparatus connected thereto and without the necessity of providing expensive or complicated additional equipment. This and other objects of my invention will appear in more detail hereinafter.

In a pr ive arrangement for an alternating current eiv trio system comprising a transformer having its neutral grounded through a ground fault neutralizer, I provide, in accordance with my invention, means for protecting the transformer against overheating by the ground fault current required for ground fault relaying of nontransitory ground faults, espectially in case of failure of the ground fault relaying protection to function and without subjecting the system to dangerous overvoltages More specifically, if the transformer is initially too hot to hazard a re c invention, such reduction by means in accordance with the heating of the transic-rmfollowing failure or" the round fault neutralize-r,

V I provid such value whereby the sys thefo-llowing description when considered duction in the impedance of the nection to ground in. of non-transitory ground faults, I prevent, in acco" Also, if the impedance reduction does occur to clear the ground fan; then the ground fault relaying fails to clear the fault before the transformer is dangerously overheated, in accordance with invention, means responsive to the heating of the former for restorin the ground connection tc-"its initial inductive valueand maintaining it at operate without being su'o ect d to dangerous overvolta-ges asmight occur wi hlthe neutral disconnected froin ground.

My invention will be better. understoc in con nection With the accompanyingdrawing and its scope will be pointed out in the appended cle Thesingle figure of the accompanying draw diagrammatically lustrates an embodiment of my invention in a ground fault neutralizer protective arrangement for a three-phase alternatin g current electric system or the typein Which r a fault to ground on a phase conductor of the system substantially increases the capacitance therewith; The circuit of the trip coil is ar-- :ranged to be controlled through the contacts it? of suitable ground fault responsive relaying means li.- 'As'illustrated, this relayingni'e a simple overcurrent relay connected to be ene gi zd in accordance with the vector sum of the currents in the phase conductcrs'i, and 3-as derived from the parallel connected transformers l-Z; l3, and respectively associated with these V conductors. The' application of other ground fault responsive relays, such as hose which are directionally responsive, will be apparentto those skilled in the art,

" Iii-"order to clear transitory or arcing ground faults wlthout interruption of service such as would follow cy'openin'g the circuit breaker 3 every time there was a ground fault on one phase conductor, suitable suppressingmeans, such as a ground fault neutralizeri l, is provided. Since the ground fault 'neutr r is usually connected to-ground through a suitably derived system neutral, there isgprovided suitable means such asa grounding transformer it having delta 'connect ed windings ll and winding ES Y-co-nnectedto.

the system phase conductors l, 2, audit to provide a neutral it. It will be apparent to those skilled in the art that an providing a neutral and relatively lowzerosequence inductance may be used. The inductive reactance or ground fault neutralizer i5; isproportioned tohave, together with the transformer inductive conn may continue nductive reactance' nection is 'd'relatively small proportion of the total inductance of this connection in order that, for groundfaults which the ground fault neutralizer can not suppress, sufficient ground fault currentima y iiowafter the neutralizer is shortcircuited topermit the operaticn'of the system groundiaultrelay ii. Also, theground 1 ,vi ded tos establish a low impedance path to transforrner is usually designed ,to have; relatively short current-time rating when it is used to ground the system directly. I

In order'that the system ground faultrelaying faults on the system, an arrangement is progroundfron i the neutral E5 in response to a predeterminedcurrentfiow for apredetermined time in the neutralizer connection through the neutral 19. For this purpose, the neutral 'lQ may be arranged to be connected substantially directly to ground by the closing of suitable switching means herein illustrated as a normally open ciruit breaker 2d of thelatchedfclosed type This circuitbrealier is provided with suitable closing means indicated as a closing coil 23 and also with suitable latch releasing means, such as a trip coil 22.

The-closin'g or" the circuit breaker it is effected by a suitable time delay relay sequence which I6, azero-sequenc'e inductance such as to provide vided by the transformer in the neutra-lizer conis initiated by theope'ration of a relay This relay is operative in dependence on the current flowing in'the'neutralizer i5 and may be connected to-be energized ironi a current transformer 24? in series relation with the neuitralizer, ass'hown: The normally closed contacts 250i the relay 23 maintain the energizing circuit ol a time delay drop-out relay 2% which may beef the induction disk alternating current energized type, examples of which are well lnic-wn'to the art. I

When the relay 26 closes its contacts 2'l'while the relay 23- is energized with its contacts closed, the circuit ofan auxiliary' relay is completed through I) auxiliary switch oont cts Sfi of the circuit breaker 23. This auxiliary relay29 through its normally closed contacts ill controls the circuit of a'second time delay re lay 32 and, when energized, also completes its own circuit through its normally open contacts 33. -When the second time delay relay drops out, the closing of its normally open contacts c5 completes the circuit of a closing relay 35 thrdugh the 11 contacts 3E vof theloircuit'loreal'rer 2.8. The closing relay35 picks up to close its normally open-seal-in contacts "at and also completes the; circuit .of the closing coil 2i through the, b auxiliary, switch contactsv 38 of the circuit breaker .With the completion or this circuit, the: closing action of the circuit breaker will by-pa'ss the groundcfault neutralizer l5 and thus reducethe impedance of the inductive connection to ground; 7

Upon the closing of the circuit breaker, 2c, the impedance oi thisccnnecticn is so reduced that enough ground ult current can flow to effect the intended o'peratien ofthe ground fault relay ll, Ii however, this ground fault relay or'the circuithrea icontrolled thereby-should fail to function, as intended and thus not disconnect th al i h amendin n forme 16 t llflls transformer would be subjected to a su tai ah current flow excc the curre aan'iej maa of the transformer. Destructinder at least serious damage to the transformer would'follow. 'In order to prevent such 2,320,149 destruction or damage, I provide, in accordance.

with my invention, means 39, the response of which is based on or reflects the heating of the grounding transformer, for restoring the neutralizer connection to its original inductive value so as to limit not only the current therein to a value which the transformer can safely carry but also to restore the system to its initially protected condition whereby to avoid dangerous overvoltages which would otherwise be possible by just merely opening the neutral-to-ground connection.

As shown, the heat responsive means 39 is a relay of the thermal type disclosed in United States Letters Patent 1,501,017, issued July 8, 1924. This relay has a thermal element comprising a thermostatic metal strip 40 and a thermal storage block 4| provided with a heating element 42. The thermostatic strip 40 and the heating element 42 are electrically connected in parallel with a suitable tapped autotransformer 43 which is energized in dependence on the zero-sequence current flowing in the grounding transformer. Thus, for example, the autotransformer 43 may be connected in circuit with a current transformer 44 in series relation with the connection to ground of the neutral l9. As will be obvious to those skilled in the art, when the grounding transformer has a closed delta winding as shown, then a current transformer in series relation therewith will also provide a current proportional to the zero-sequence current ilowing in the grounding transformer. The relay 39 also includes a cold contact 45 and a hot contact 46 through which circuits can be made or broken by the thermostatic strip 45.

Although for the purpose of illustrating my invention I have shown a specific thermally responsive relay, it will be apparent to those skilled in the art that other types of thermal relays can be used so long as they respond in accordance with the heating of the transformer. Broadly speaking, any relay whose time of contact controlling action is inversely proportiona1 to the square of the zero-sequence current flowing in the ground connection through the neutral l9 will function in accordance with the heating of the grounding transformer l6. Thus, for example, an inverse time element induction type of motor relay whose rotor torque is dependent upon the square of the energizing current as obtained, for example, from the current transformer 44 may be employed. Examples of such relays are well known to the art. One advantage, however, accrues from the use of a strictly thermal type of relay in that by mounting it against or in the transformer tank the temperatures of both the transformer and the relay are initially substantially the same.

Now, in accordance with my invention, I so arrange that if the heating of the grounding trans former l6 tends to exceed the current-time rating of the transformer while the switch is closed, this switch will be immediately opened to restore the impedance of the inductive connection to its initial inductive or capacitance current suppressing value. For this purpose, I so arrange that the closing of the contact 46 by the thermostatic strip 40 effects the energization of the trip coil 22. In order to do this and yet take care of the fairly large current drawn by the trip coil, there may be provided an auxiliary relay 41, the circuit of which is controlled through the hot contact 46 of the thermal relay and a auxiliary switch contacts 48 of the switch 20. The relay 41 includes circuit closing seal-in contacts 49 and circuit closing contacts 50 in the circuit of the trip coil 22 whereby to effect the energization of this trip coil if the circuit breaker 20 is closed.

Inasmuch as the opening of the switch 20 restores the neutralizer connection to its initial condition, the resultant flow of lagging current in this connection would again start the relaying sequence by Which the switch 20 is closed. In order to prevent an undesired repetition of such closing during the continuance of a permanent fault which the ground fault relay means I I and the circuit breaker 1 have been unable to dis connect from the grounding transformer I6, I further provide, in accordance with my invention, means for preventing a repetition of the closing of the switch 20 while the grounding transformer is excessively heated. While this can be done in different ways, I have shown an arrangement for preventing the time delay relay sequence operation by which the closing of the switch 20 is effected. Thus, for example, I provide an auxiliary relay 5| arranged to control normally closed contacts 52 in series with the winding of the auxiliary relay 29. The circuit of the relay 5| is controlled through the hot contact 45 of the thermal relay 39 so that as soon as the grounding transformer I6 is excessively heated, the relay 5| will be energized to open its normally closed contacts 52 and seal itself in through its normally open contacts 53. The opening of the contacts 52, of course, eliminates any further possibility of closing the switch 29 until the relay 5| is restored to its normally deenergized condition. This can be accomplished by the opening of a normally closed manually operated switch 54.

Since it is possible that under some abnormal system conditions the grounding transformer may become unduly heated, I may also provide, in accordance with my invention, means for preventing the initial closing of the switch 2|] in the event that the transformer is already too warm before this occurs. For this purpose, I provide, as shown, an auxiliary relay 55 which, as long as it is energized, maintains its contacts 56 closed in the circuit of the auxiliary relay 29. Thus, as long as the contacts 56 of this relay and the contacts 52 of the relay 5| are closed, the closing operation of the switch 20 may occur. In accordance with my invention, I control the energization of the auxiliary relay 55 by the cold contact 45 of the thermal relay means 39. Then, if the grounding transformer I6 is sufliciently cool, the cold contact 45 will be closed to complete the circuit of the relay 55 and thus maintain its contacts 56 closed. If, on the other hand, the cold contact 45 is open when a ground fault occurs on the line or if it opens before the switch 20 can be closed by the sequencing relay operation, then the circuit of the auxiliary relay 29 will be broken at the contacts 56 of the relay 55, and the sequencing relay operation can not be carried out. Accordingly, the switch 20 can not be closed until such time as the thermostatic strip 40 engages the cold contact 45. While I have shown the contacts 52 of the auxiliary relay 5| and the contacts 56 of the auxiliary relay 55 in the circuit of the auxiliary relay 29, it will be obvious to those skilled in the art that all that is required is that the auxiliary relays 5| and 55 prevent the closing of the switch 25 when the transformer is not sufficiently cool, and that these contacts may be placed elsewhere to prevent such closing action.

.While. 1 have shown and g ce I bleiq 1 1; I, gq v 88 115129 he ted, to the, exact 'aitrngen enii hawn,

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em a la r be en vwi e v is 921.- e ,v. .sl pend lm 'th system to 9% 12 @urrqn 110M138 inseid iusiu iive 9922399: pmyicle. a neuiral and a reactanse device conreq iqe a Q t e @iter. meecmrrem aeei pi between. saidnemral. and. gmune and taingecilrientfte grujid at t'iie'grunded point and tfve r'eactance, means for reducin'gtiae' inipeti-- rence 'ofa ground fault; which is not s'fippressed having a relatively high zero-sequence inductance compared to the zero-sequence inductance of said inductive reactance means, means for reducing the impedance of said inductive connection operative in dependence on the current flowing in the connection a predetermined timeafter the occurrence of a ground fault witch is not suppressed by the lagging current flowing in the connection, means responsive to the increased flow of ground current in consequence of the reduction of the impedance of said inductive conmotion for effecting an interruption in the phase conductors of said system between the fault and the point of connection of said inductive reactance means to the system, means thermally responsive in accordance with the heating of said inductive reactance means in consequence ofthe increased flow of ground current therein for restoring said inductive connection to its original inductive value within the current-time rating of said inductive reactance means whereby to prevent the destruction thereof in case of the failure of said increased ground current flow responsive means to function within said current-time rating, and means controlled by said heating responsive means for preventing a reduction in the impedance of said inductive connection unless the heating of said inductive reactance means is below a predetermined amount.

5. In an alternating current electric system of the type wherein a fault to ground on one phase conductor of the system substantially increases the capacitance current to ground of the ungrounded phase conductors of the system, a zerosequence inductiv connection to ground proportioned to provide on the occurrence of a round on a phase'conductor of the system a lagging current for effectively suppressing the capacitance current to ground at the grounded point and comprising inductive reactance means connected to the phase conductors of the system to provide a neutral and a reactance device connected between said neutral and ground and having a relatively high zero-sequence inductance compared to the zero-sequence inductance of said inductive reactance means, means for reducin the impedance of said inductive connection operative in dependence on the current flowing in the connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in the connection, means responsive to the increased flow of ground current in consequence of the reduction'of the impedance of said inductive connection for effecting an interruption in the phase conductors of said system between the fault and the point of connection of said inductive reactance means to the system, means thermally responsive in accordance with the heating of said inductive reactance means in consequence of the increased flow of ground current for restoring said inductive connection to its original inductive value within the current-time rating of said inductive reactance means whereby to prevent the destruction thereof in case of the failure of said increased ground current flow responsive means to function within said current-time rating, and

means controlled by said heating responsive means for preventing a subsequent operation of said impedance reducing means.

6. In an alternating current electric system of the type wherein a fault to ground on one phase conductor of the system substantially increases the capacitance current to round of the ungrounded phase conductors of the system, a zerosequence inductive connection to ground proportioned to provide on the occurrenc of a ground on a phase conductor of the system a lagging current for effectively suppressing the capacitance current to ground at the grounded point and comprising inductive reactance means connected to the phase conductors of the system to provide a neutral and an inductive reactance device connected between said neutral and ground and having a relatively high zero-sequence inductance compared to the zero-sequence inductance of said inductive reactance means, means for reducing the impedance of said inductive connection operative in dependenc on the current flowing in the connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in the connection, means responsive to the increased flow of ground current in consequence of the reduction of the impedance of said inductiv connection for effecting an interruption in the phase conductors of said system between the fault and the point of connection of said inductive reactance means to the system, and means thermally responsive in accordance with the heatin of said inductive reactance means in consequence of the increased flow of ground current therein for restoring said inductive connection to its initial inductive value within the current-time rating of said inductive reactance means whereby to prevent the destruction thereof in case of the failure of said increased ground current flow responsive means to function within said currenttime rating.

'7. In an alternating current electric system of the type wherein a fault to ground on one phase conductor of the system substantially increases the capacitance current to ground of the ungrounded phase conductors of the system, a zerosequence inductive connection to ground proportioned to provide on the occurrence of a ground on a phase conductor of the system a lagging current for effectively suppressing the capacitance current to ground at the grounded point and comprising inductive reactance means connected to the phase conductors of the system to provide a neutral and a reactance device connected between said neutral and ground and having a relatively high zero-sequence inductance compared to the zero-sequence inductance of said inductive reactance means, means for reducing the impedance of said inductive connection operative in dependence on the current flowing in the connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in the connection, and means thermally responsive in accordance with the heating of said inductive reactance means when the heating thereof exceeds a predetermined amount prior to the expiration of the predetermined time of operation of said impedance reducing means to prevent the operation thereof.

8. In a three-phase alternating current electrio system of the type wherein. a fault to ground on one phase conductor of the system substan tially increases the capacitance current to ground of the ungrounded phase conductors of the system, a zero-sequence inductive connection to ground proportioned to provide on the occurrence of a ground on a phase conductor of the system a lagging current for effectively suppressing the capacitance current to ground at the grounded point and comprising a transformer having a set of delta-connected windings and a set of windings connected in Y to the phase conductors of the system and a reactance device connected .he-

tween the neutral of said Y-cormected windings and ground and having a relatively vhigh zerosequence inductance corn-pared to .the :zero-' I scquenceinductance or" said transformer, means for connecting the neutral of said V-connected windings substantially directly to groundcoinprising alsvvitch and means operative in depend ence on the currentflowing in inductive connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in'the inductive connection for shooting the closing of said switch; means responsive to theincreased flow of ground current in consequence of the closing of said'sw'itch for effecting an interruption oi the phase conductors. of thesystem between the 4 fault and the point of connection of saicl'Y-c'onnected tojt-he system, a thermal relay responsivein accordance with the heating of said transformer in consequence of the increased'fiow of groundcurrent for efiecting the opennigof said switch the curreut-time rating of the transformer whereby to prevent the'destruction' thereof in case of theiailure' of said increased ground current flow responsive means to function withinsaid current-time rating, and a relay controlledby said thermal relay for preventing thereclosing of said switch when said increased ground fault flow responsive means fails to function. a

9. In" athree-phasealternating current electrio system of the type wherein a fault to ground on one phase conductor of the systemsubstan tially increasesthe capacitance current to ground of the unground'ed phase conductors of the system, a zero-sequence inductive connection to ground proportioned to provide on the occurrence of a ground on aphase conductor of the system a lagging current for eiiectively suppressing the capacitance current to ground at the grounded point and comprising a transformer having a" set .of delta-connected windings and a setof windings ccnnecte'd'in Y to the phase condoctors of the system; anda reactance device connected between the neutral point ofsaid Y- connected windings and ground and having a rent dovringin the inductive connection for effectingtheclosing of said switch, meansrespom sive to the increased flow of ground current in consequence of the closing'ofsaid switch foreffecting an interruption of the phase conductors of said system between the fault and the point of connection of saidY-connected windings to the system, a thermal relay responsive in'accordshoe with the heating of said grounding transformer in consequenceoi the increased flow of ground current for effecting the opening of said switch within the currenttiiue rating of the transformer whereby to prevent the destruction of the transiormerjin case of thefailure of said increased ground current flow re sponsive means to function within said current-time rating, and means controlled by said thermal relay for preventing a second closingof said switch solelyiin accuses r dependence'on the current flowing in saidconneotion.

7,10. In a three-phase alternating current electric system of the type wherein a fault to ground on one phase conductor of the system substantially increases the capacitance current to ground of the ungrounded phase conductors of the systern, a zero-sequence inductive connection to ground proportioned to provide on the occurrence of a ground on a phase conductor of the system a lagging currentior effectively supprcssingthe capacitance currentto ground at the-grounded point-andv comprising a. grounding transformer having a set of delta-connected windings and a set of win-dings connected in Y to the phase conductors of the system, and a reactance device connected between the neutral point of said fconnectedwindings and ground and having a relatively high zero-sequence inductance compared to'the zero-sequence inductance of said grounding transformer, means for connecting the neutral point of said f r-connected windings substantially directly to ground comprising a vswitch and means operative independence on the current flowing in saidinductive connection a predetermined time after the occurrence of a ground'fault which is not suppressed by the lagging current flowing in the inductive connection for eifecting the closing of said switch, means responsive to the increased flow of ground current in consequence of the closing of said switch for efiecting an interruption oftlie phasecom ductors of said system between the'iault and-the point of connection of said Y c'onne'cted Windings to the system, and athermal' relay responsive to the heating of said groundingtransformer in consequence of the increased flow of ground current for eiiecting theopening of saidswitch within the current-time'rating oi the transformer whereby to prevent the'destruction of the transformer in case of the failure of said increased ground current flow responsive means to function Withinsaid current-time rating, V

11. In a three phase alternating current electrio system. of the type wherein a fault" to ground on onephase conductor of the systemsubstantially increases the capacitance current to'ground of the ungrounded phase conductors of'the system, a zero-sequence inductive connection to groundproportioned to provide on the occurrence of aground on a phase conductor of the system a lagging current for: effectively suppressing; the capacitance current to ground at the grounded point and comprising a transformer having windings connected to the-phase condoctors of the system to provide a neutral, and

a vreactance device connected between said neutral and'ground and havinga relatively high zero-sequence inductance compared to the zerosequence inductance of said transformer, means for connecting the neutralcf Said transformer substantially directly to ground comprising a switch, means operative in dependence on the current flowing in said inductive connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in the inductive connection for effecting the closing of said switch, means responsive to the increased flow of ground ourrent in consequence of the closing of said'switch for effecting an interruption of the pha'se cone ductors ofthe system between the fault and the point'ofi-c-onnection of said transformer windings .to the system, a thermal relay responsive in ac-,

cordance With the heating of said-transformer in consequence of the increased flow of ground current for effecting the opening of said switch within the current-time rating of the transformer whereby to prevent the destruction thereof in case of the failure of said increased ground current flow responsive means to function within said current-time rating.

12. In a three-phase alternating current electrio system of the type wherein a fault to ground on one phase conductor of the system substantially increases the capacitance current to ground of the ungrounded phase conductors of the system, a zero-sequence inductive connection :to ground proportioned to provide on the occurrence of a ground on a phase conductor of the system a lagging current for effectively suppressing the capacitance current to ground at the grounded point and comprising a transformer having windings connected to the phase conductors of the system to provide a neutral, and a reactance device connected between said neutral and ground and having a relatively high zero-sequence inductance compared to the zero-sequence inductance of said transformer, means for connecting the neutral of said transformer substantially directly to ground comprising a switch, and means operative in dependence on the current flowing in said inductive connection a predetermined time after the occurrence of a ground fault which is not suppressed by the lagging current flowing in the inductive connection for eifecting the closing of said switch, means responsive to the increased flow of ground current in consequence of the closing of said switch for effecting an interruption of the phase conductors of the system between the fault and the point of connection of said transformer windings to the system, a thermal relay responsive in accordance with the heating of said transformer in consequence of the increased flow of ground current for effecting the opening of said switch within the current-time rating of the transformer whereby to prevent the destruction thereof in case of the failure of said increased ground current flow responsive means to function within said currenttime rating, and a relay controlled by said thermal relay for preventing the reclosing of said switch when said increased ground fault flow responsive means fails to function.

PHILIP H. LIGHT.

CERTIFICATE OF CORRECTION.

Patent No. 2,526,1L 9. August 10, 1915.

PHILIP H. LIGHT.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, second column, line 51, for "espectially'" read especially-; Page 2, first column, line 6h, for "and' read -any--; page 5, second column, line 20, strike out the period after "ways" and insert instead a comma; pa e 1;, secsecond column, line 67, claim i for "connected" read -connection; and that the said Letters Patent should be read with this correction therein that the same may conform t o the record of the case in the Patent Office. Signed and sealed this 12th day of October, A. D. 1915.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents.

CERTIFICATE O CORRECTION. Patent No. 2,526,1L 9. August 10, 191

PHILIP H. LIGHT.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, sec- 0nd column, line 51, for "espectially read -especially-; Page 2, first column, line 6L for "and" read "any"; page 5, second column, line 20,

strike out the period after "ways" and insert instead a comma; page 1;, sec-- second column, line 67, claim 1;, for "connected" read -connection-; and that the said Letters Patent should be read with this correction therein that the same may conform t o the record of the case in the Patent Office.

Signed and sealed this 12th day of October, 19LL5.

Henry Van Arsdale,

(Seal) Acting Commissioner of Patents. 

